The chemokine receptors CCR5 and CXCR4 are the predominant coreceptors for HIV-1 in vivo, and all HIV-1 strains are currently classified as R5, X4, or R5x4.1 As members of the 7 transmembrane-domain G protein-coupled receptor (GPCR) superfamily, CCR5 and CXCR4 share common structural features, including an extracellular N-terminus, 3 extracellular domains, 3 intracellular domains, and an intracytoplasmic C-terminal tail. The functions of CCR5 and CXCR4 as chemokine receptors and HIV-1 coreceptors are separable, in that the binding sites for their biologic ligands (ie, chemokines) and for HIV-1 gp120 were found to be discrete with some overlap.2-5 Thus, binding of a given protein or small molecule to the HIV-1 coreceptor is not necessarily predictive of HIV-1 inhibition.
Previously, it was reported that a soluble tachyzoite extract of the protozoa Toxoplasma gondii (STAg) uses the chemokine receptor CCR5 on murine dendritic cells to induce interleukins-12 (IL-12) production.6 The active component in STAg that signals through CCR5 was recently identified as cyclophilin-18(C-18).7 
HIV infection and AIDS provide a growing medical problem for health care professionals around the world. New and effective treatments are needed to slow the spread, and eventually eradicate, the disease.